The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the line pressure in a hydraulic circuit for the transmission.
A known control system for a continuously variable belt-drive transmission disclosed in EP-A-No. 205 257 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to control the transmission ratio to a desired transmission ratio which is determined in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure to a necessary line pressure which is determined in accordance with the transmission ratio and the engine torque. The output of the engine is transmitted to the drive pulley through a clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (upshift). At that time, if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced at a speed which is determined by the line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio.
In such a system, it is preferable to shift the spool of the line pressure control valve by control oil supplied to an end of the spool and to control the line pressure by controlling the amount of the control oil. The control of the amount of the control oil is carried out by controlling the drain of the control oil by a solenoid operated valve which is intermittently operated by pulses. Thus, by controlling the duty ratio of the pulses, the line pressure is controlled to a necessary value.
However, if the duty ratio of the pulses does not properly correspond to the desired ratio, the line pressure can not be precisely and quickly controlled to the necessary value.